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Additive metallurgy - Thermal influences on structure and properties of stainless steel 316L
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).ORCID iD: 0000-0002-0335-8927
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Additive manufacturing (AM) as a manufacturing process has, in recent years, become widely accepted as capable of manufacturing parts that will be used in end products. In this thesis, stainless steel grade 316L parts are manufactured using two different powder bed fusion techniques, selective laser melting (SLM) and electron beam melting (EBM). It is recognized that parts made using these processes will have unique microstructures and mechanical properties that have not been seen in bulk parts produced with other methods. The driving force behind the formation of these structures is the fast cooling speed that induces segregation of elements, forming an inhomogeneous microstructure. How these structures react to thermal treatment is less well known and an essential aspect in many applications. Parts manufactured using SLM was treated with hot isostatic pressing (HIP) to investigate if the material retains its unique features. Two different HIP cycles were used, one with 1150 °C and one with 1040 °C. In both cases, the cellular sub-grain structure fades. The cycle utilizing the high temperature is found to coarsen the grain structure and thus lowering the mechanical properties significantly. As manufactured parts show yield strength (615±1 MPa), tensile strength (725±2 MPa) and microhardness (211±10 Hv), compared to values of yield strength (284±2 MPa), tensile strength (636±1 MPa) and microhardness (178±8 Hv) after 1150 °C HIP. Using HIP at 1040 °C, the material will retain a finer grain structure resulting in higher yield strength (417±7 MPa) compared to 1150 °C HIP temperature, while the UTS and hardness have a similar value. It is also observed that the dispersed inclusions formed during SLM are still present after HIP to increase the mechanical properties compared to a conventionally annealed bar (TS: 515 MPa, YS: 205 MPa). Samples manufactured using EBM was investigated to understand the influence of the in-situ heat treatment that is present in the EBM process. The material possesses a long-range heterogeneous structure in addition to the cellular structure, where the cellular structure is present at the top and disappears further down the sample. Samples with different geometries were produced to study the effect of heat flux, cooling speed and the elevated temperature of 800 °C that is present during the EBM process. The thickness of the cell boundaries is measured in different areas, revealing that geometry and size of manufactured parts have a significant impact on the evolving microstructure. It is also revealed that the tensile strength (562±4 MPa) and microhardness (161±11 Hv) is not affected by the change in microstructure, resulting in a very homogeneous material concerning these parameters. Heat treating the material at 800 °C show that the cellular structure becomes diffuse after several hours, but the grain morphology stays the same.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry, Stockholm University , 2020.
Keywords [en]
Additive manufacturing, Selective laser melting, Electron beam melting, Hot isostatic pressing, Stainless steel, Microstructural heterogeneity
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-176458ISBN: 978-91-7797-968-5 (print)ISBN: 978-91-7797-969-2 (electronic)OAI: oai:DiVA.org:su-176458DiVA, id: diva2:1376249
Public defence
2020-02-07, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 09:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 5: Manuscript. Paper 6: Manuscript.

Available from: 2020-01-15 Created: 2019-12-09 Last updated: 2020-01-17Bibliographically approved
List of papers
1. Hierarchical structures of stainless steel 316L manufactured by Electron Beam Melting
Open this publication in new window or tab >>Hierarchical structures of stainless steel 316L manufactured by Electron Beam Melting
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2017 (English)In: Additive Manufacturing, ISSN 2214-8604, Vol. 17, p. 106-112Article in journal (Refereed) Published
Abstract [en]

One of the serious obstacles preventing wide industrial use of additive manufacturing (AM) in metals and alloys is a lack of materials available for this technology. It is particularly true for the Electron Beam Melting (EBM®) process, where only a few materials are commercially available, which significantly limits the use of the method. One of the dominant trends in AM today is developing processes for technological materials already widely used by other methods and developed for other industrial applications, gaining further advantages through the unique value added by additive manufacturing. Developing new materials specifically for additive manufacturing that can utilize the properties and specifics of the method in full is still a research and development subject, and such materials are yet far from full scale industrial usage. Stainless steels are widely used in industry due to good mechanical properties, corrosion resistance and low cost of material. Hence, there is potentially a market for this material and one possible business driver compared with casting for example is that lead times could be cut drastically by utilizing an additive approach for one-off or small series production. This paper presents results from the additive manufacturing of components from the known alloy 316L using EBM®. Previously the samples of 316L were made by laser-based AM technology. This work was performed as a part of the large project with the long term aim to use additively manufactured components in a nuclear fusion reactor. Components and test samples successfully made from 316L stainless steel using EBM® process show promising mechanical properties, density and hardness compared to its counterpart made by powder metallurgy (hot isostatic pressing, HIP). As with the other materials made by EBM® process, 316L samples show rather low porosity. Present paper also reports on the hierarchical microstructure features of the 316L material processed by EBM® characterized by optical and electron microscopy.

Keywords
Electron beam melting, Stainless steel, 316L, Characterization, Microstructures
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-176454 (URN)
Available from: 2019-12-06 Created: 2019-12-06 Last updated: 2019-12-11Bibliographically approved
2. Micro- and macro-structural heterogeneities in 316L stainless steel prepared by electron-beam melting
Open this publication in new window or tab >>Micro- and macro-structural heterogeneities in 316L stainless steel prepared by electron-beam melting
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2018 (English)In: Materials Characterization, ISSN 1044-5803, E-ISSN 1873-4189, Vol. 141, p. 1-7Article in journal (Refereed) Published
Abstract [en]

This is a study of the micro- and macrostructural variations in samples of stainless steel with the overall composition of the grade 316L, produced using electron beam melting. Electron beam melting is one of the processing methods under consideration for manufacturing some of the International Thermo- Nuclear Experimental Reactor In-Vessel components. Therefore further studies of the homogeneity of the material were conducted. Electron beam melting results in a complicated thermal history of the manufactured part giving a significant impact on the microstructure. A cellular structure that is often observed in samples prepared by selective laser melting was found in the top layers of the specimens. Further down, the structure changed until the cellular structure was almost non-existing, and the grain boundaries had become more pronounced. This revelation of a heterogeneous structure throughout the entire part is crucial for large-scale industrial applications like the Thermo- Nuclear Experimental Reactor to make sure that it is understood that the properties of the material might not be the same at every point, as well as to assure that the correct post-treatment is done. It is also exposed that a significant part of this change is due to molybdenum redistribution inside the sample when it diffuses from the cell boundaries into the cells, and into bigger agglomerates in the grain boundaries. This diffusion seems not to affect the microhardness of the samples.

Keywords
Additive manufacturing, Electron beam melting, Microstructure, 316L stainless steel, Heterogeneous material
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-158385 (URN)10.1016/j.matchar.2018.04.026 (DOI)000435428100001 ()
Available from: 2018-08-14 Created: 2018-08-14 Last updated: 2019-12-11Bibliographically approved
3. Tailoring hierarchical structures in selective laser melted materials
Open this publication in new window or tab >>Tailoring hierarchical structures in selective laser melted materials
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2017 (English)In: IOP Conference Series: Materials Science and Engineering, ISSN 1757-8981, E-ISSN 1757-899X, Vol. 219, article id 012036Article in journal (Refereed) Published
Abstract [en]

With selective laser melting the potential to manufacture a wide variety of geometries from different materials has presented itself. Interest in this technology keeps growing every year, and with that growth a deeper understanding of the process and resulting materials is urgently needed. In this paper we present a short overview of the structural elements that appear during selective laser melting, and explain how to tailor them to achieve specific structures and material properties. Melt-pools, texture and grains, subgrain cells, and inclusions are the elements discussed herein, and tailoring of these elements can have effects on density, and corrosion resistance, as well as mechanical properties in general.

National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-176452 (URN)10.1088/1757-899X/219/1/012036 (DOI)
Conference
38th Risø International Symposium on Materials Science, Risø, Norway, 4-8 September, 2017
Available from: 2019-12-06 Created: 2019-12-06 Last updated: 2019-12-09Bibliographically approved
4. 316L stainless steel designed to withstand intermediate temperature
Open this publication in new window or tab >>316L stainless steel designed to withstand intermediate temperature
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2017 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 135, p. 1-8Article in journal (Refereed) Published
Abstract [en]

Austenitic stainless steel 316L was fabricated for withstanding elevated temperature by selective laser melting (SLM). Tensile tests at 800 degrees C were carried out on laser melted 316L with two different strain rates of 0.05 S-1 and 0.25 S-1. The laser melted 316L showed tensile strength of approximately 400 MPa at 800 degrees C, which was superior to conventional 316L. Analysis of fracture surface showed that the 316L fractured in mixed mode, ductile and brittle fracture, with an elongation of 18% at 800 degrees C. In order to understand the mechanical response, laser melted 316L was thermally treated at 800 degrees C for microstructure and phase stability. X-ray diffraction (XRD) and Electron back scattered diffraction (EBSD) of 316L treated at 800 degrees C disclosed a textured material with single austenitic phase. SEM and EBSD showed that the characteristic and inherent microstructure of laser melted 316L, consisting of elongated grains with high angle grain boundaries containing subgrains with a smaller misorientation, remained similar to as-built SLM 316L during hot tensile test at 800 degrees C. The stable austenite phase and its stable hierarchical microstructure at 800 degrees C led to the superior mechanical response of laser melted 316L.

Keywords
Selective laser melting, Thermo-mechanical processing, Mechanical properties, Microstructure evolution
National Category
Materials Engineering Materials Chemistry
Identifiers
urn:nbn:se:su:diva-148810 (URN)10.1016/j.matdes.2017.08.072 (DOI)000413236300001 ()
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2019-12-09Bibliographically approved
5. Investigation of the heterogeneous structure and its effect on mechanical properties of electron beam melting fabricated 316L stainless steel
Open this publication in new window or tab >>Investigation of the heterogeneous structure and its effect on mechanical properties of electron beam melting fabricated 316L stainless steel
(English)Manuscript (preprint) (Other academic)
Abstract [en]

316L stainless steel samples were prepared with Electron Beam Melting. Samples were produced to investigate what parameters and conditions govern the microstructural evolution during the process, as well as the effect of the microstructural variations on the tensile strength. The effects investigated in this study are the cooling speed of the melt, the heatwaves moving through the sample due to additional layers being melted on top, and the constant base temperature of around 800 °C. Several different samples were prepared to simulate different conditions that occur during the manufacturing of real parts. The microstructures of these samples were investigated and characterized using scanning electron microscope. The investigation showed that all these effects had different effects on the microstructure. The tensile strength was homogeneous throughout the material, with little variance in the values. The ultimate tensile strength was recorded to 562±4 MPa, the yield strength to 261±3 MPa, and the Elongation to 67.5±4.8 %. Further investigation into why the change in microstructure has no impact on tensile strength was done using transmission electron microscopy. It was revealed that the material was missing the dislocation networks that are responsible for the increased strength observed in samples prepared by the selective laser melting process.

National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-176464 (URN)
Available from: 2019-12-06 Created: 2019-12-06 Last updated: 2019-12-11Bibliographically approved
6. An investigation into the effect of Hot Isostatic pressing on parts manufactured with Selective Laser melting
Open this publication in new window or tab >>An investigation into the effect of Hot Isostatic pressing on parts manufactured with Selective Laser melting
(English)Manuscript (preprint) (Other academic)
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-176465 (URN)
Available from: 2019-12-06 Created: 2019-12-06 Last updated: 2019-12-11Bibliographically approved

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